How do different layer production or housing systems affect food safety, including the risk of salmonella in eggs?
USDA and North Carolina State researchers compared conventional cages to "enriched" colony-type cages. They used hens experimentally infected with two types of Salmonella Enteritidis, which is a leading cause of human salmonellosis. Overall, housing in conventional cages resulted in higher frequencies of salmonella in the birds' livers, spleens, ovaries, and oviducts. Because different layer housing systems influenced the pathogen's colonization of egg-producing organs at different rates, there may be food safety implications. However, the researchers cautioned that "the mechanisms by which housing systems might exert such effects remain undetermined and await further inquiry."
Eggs contaminated with Salmonella enterica subspecies enterica serovar Enteritidis (Salmonella Enteriditis) cause more human illnesses worldwide than any other type of food contaminated with this pathogen. Contamination occurs following systemic infection in layers, which leads to the bacteria colonizing reproductive organs, including the ovaries and oviducts.
Different layer production or housing systems may offer a variety of animal welfare or public health advantages. Yet, when it comes to food safety, many questions remain about the effects of different production systems.
Decades of salmonella on-farm risk reduction programs are reducing the incidence of human infections around the world. Even so, pathogenic bacterial strains with greater ability to cause systemic infection in poultry -- like Salmonella Enteriditis -- still emerge from the "environmental reservoirs" of rodent and insect populations in and around poultry houses. As a result, although the pathogen's prevalence inside commercially produced eggs may be very low, the risk of food poisoning persists. Epidemiologic analyses and active disease surveillance efforts continue to document a public health threat from egg-transmitted salmonellosis.
As for layer flock housing, scientific studies have not yet established a consistent pattern of pathogen prevalence in either of two widely used systems -- conventional cage housing versus "enriched" or furnished, colony-type cages.
In this study, a team from the USDA's Georgia-based Egg Safety and Quality Research Unit and the Department of Poultry Science at North Carolina State University collaborated "to determine the effects of the two housing systems on the invasion of internal organs by Salmonella Enteritidis in experimentally infected laying hens."
The researchers conducted two similar trials using 29- and 34-week-old specific-pathogen-free (SPF) Single Comb White Leghorn hens (n = 124) from the Southeast Poultry Research Laboratory. They distributed the hens into four separately housed groups in different rooms of a disease-containment facility equipped with the two types of cage systems simulating commercial conditions. In the conventional housing, each cage housed six hens with floor space of 648 cm2 (100 sq. in.) per hen. In the enriched housing, each cage housed 16 hens with floor space of 1,216 cm2 (188 sq. in.) per hen, including access to perches, an enclosed nesting area, and a scratching area.
All hens received ad lib water (by automatic nipple drinkers) and pelleted feed. The research team noted that stocking densities in both housing systems represented two-thirds of the maximum levels recommended by the cage manufacturer.
In both trials, researchers orally inoculated hens with a measured dose of Salmonella Enteritidis. Trial 1 hens received a phage type 13a isolate, while those in Trial 2 received a phage type 4 isolate. Researchers collected fecal samples pre-inoculation, then at five days or six days post-inoculation. Then they euthanized hens and collected organ samples, including portions of ceca, liver, spleen, ovary, and oviduct.
No fecal samples were positive for salmonella pre-inoculation, which confirmed that all hens were SPF at the start. For Trial 1 hens (inoculated with phage type 13a), nearly 97% had ceca positive for Salmonella Enteritidis at five or six days post-inoculation. For Trial 2 hens (inoculated with phage type 4), about 94% had infected ceca. There was no significant difference between layer housing systems for ceca infection.
However, in other organs -- liver, spleen, oviduct -- there was greater Salmonella Enteritidis phage type 13a (Trial 1) in hens housed in conventional cages. The higher frequencies of these organ infections also occurred in phage type 4 inoculated hens and included greater infection of ovaries as well.
The researchers concluded: "Housing type (conventional or enriched) significantly influenced the frequency of Salmonella recovery from all four internal organ sites (when considering the bacterial strains together), as consistently more positive results were obtained from the tissues of hens in conventional cages."
What does this study mean for producers?
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Housing type -- conventional or enriched -- can influence levels of human pathogenic salmonella in the organs of laying hens, including oviducts where eggs form
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Conventional housing -- which typically has higher stocking density compared to enriched housing -- may present higher risk for Salmonella Enteritidis infection
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Additional research may pinpoint characteristics of housing that help mitigate or reduce salmonella in layers
The full paper, titled "Colonization of internal organs by Salmonella Enteritidis in experimentally infected laying hens housed in conventional or enriched cages" can be found in Poultry Science and online here.
DOI: 10.3382/ps.2012-02811